Low coefficient of friction silicone release formulations incorporating higher alkenyl-functional silicone gums

ABSTRACT

A curable coating composition comprising an ethylenically unsaturated organopolysiloxane base polymer, an organohydrogenpolysiloxane crosslinking agent, an effective amount of a precious metal containing hydrosilylation catalyst, and from 0.1 to 20 weight of a higher alkenyl-functional organopolysiloxane gum. The incorporation of the gum provides a composition which cures rapidly to a low coefficient of friction coating having stable release over time.

TECHNICAL FIELD

This invention relates to curable silicone release formulations that arecurable to coatings that exhibit both stable release from a broad rangeof pressure-sensitive adhesives and low coefficients of friction.

BACKGROUND

Silicone release formulations are known and find utility in a variety ofapplications. Generally they are applied to a surface from either asolvent-based composition or a solvent-free composition. Solvent-basedcompositions typically result in cured coatings which exhibit asignificantly lower coefficient of friction (COF) than do solvent-freecompositions. While it is not fully understood why this is so, it isbelieved that it is at least partially due to the higher molecularweight of the segments between the functional groups present in thesolvent-based compositions.

Solvent-free systems are usually based on relatively highlyfunctionalized silicone polymers in the 2,000 to 30,000 molecular weightrange and typically yield densely crosslinked, very rubbery feelingcoatings. Formulations designed for solvent delivery, on the other hand,normally contain polymers with molecular weights in excess of 100,000containing relatively fewer functional groups as the major siliconecomponent. Cured coatings of these systems will have a much highermolecular weight between crosslinks, and the motion and flexibility ofthese long, unrestricted lengths of polydimethylsiloxane are believed tobe largely responsible for the much more slippery feel of thesecoatings.

Typically, COF values for solvent-cast and solvent-free coatings are0.05 and 0.40, respectively. These values are measured on a curedcoating applied to polyethylene coated kraft paper at a coating weightof 1 g/m².

For many applications, such as those requiring hand application of atransfer adhesive, higher COF is a serious disadvantage. For example, insuch an application the user typically applies one face of the adhesiveto a surface with a release liner in place on the second exposedadhesive surface. He or she then applies pressure to the release linerto secure the adhesive to the surface. The release liner may then beremoved to expose the second face of adhesive.

In applying pressure to the release liner, the user often encountersresistance due to friction. If this resistance is too great, he or shemay develop calluses or blisters on his or her fingers. It is desirable,therefore, to minimize this resistance. This may be accomplished byreducing the COF. While solvent-based compositions do provide a low COF,they require the use of special equipment to remove and recover thesolvent. This undesirably adds to the complexity and cost of processingthese systems.

Solvent-free systems, while not requiring the use of this specialequipment, typically do not provide the low COF needed. Thus, they donot fulfill the need of the user.

A variety of approaches have been tried in order to obtain a low COFsolvent-free silicone release coating. For example, Japanese KOKAI SHOWA61-159480 (published Jul. 19, 1986) discloses a solvent-free, additioncurable composition in which a few weight percent (wt %) of a highmolecular weight vinyl functional silicone gum is incorporated in theformulation. For this approach to succeed, it is critical that the addedgum incorporate a significantly lower level of functionality than thebase polymer of the host formulation. If this condition is met, the gumapparently is preferentially expressed at the surface of the coatingduring the curing process leading to reductions in the COF.

Other approaches to reducing COF in addition curable formulations thathave been described include the use of both branched siloxanes (see U.S.Pat. No. 5,082,951) and siloxanes having a trivinylsiloxy group at oneend of the base polymer (see U.S. Pat. No. 4,870,149). The in situgeneration of branched siloxanes in addition cure systems as a means ofachieving reductions in COF, has been disclosed in Japanese KOKAI SHOWA63-101453 and the in situ synthesis of siloxane gums has been disclosedin European Patent Publication No. 0 446 030 A2. The addition of vinylfunctional gums to addition curable release formulations as cureaccelerators has been disclosed in Re. 31,727.

As used in the prior art, a vinyl-functional polymer or gum is one inwhich the CH₂ ═CH-- groups are attached directly to the silicon atoms ofthe organopolysiloxane polymer or gum. Although the use ofvinyl-functional gums does reduce the COF, it also reduces the rate ofcure in higher alkenyl-functional systems.

SUMMARY OF THE INVENTION

The present invention provides a curable coating composition comprising

(a) an ethylenically unsaturated organopolysiloxane base polymer;

(b) an organohydrogenpolysiloxane crosslinking agent;

(c) an effective amount of a precious metal-containing hydrosilylationcatalyst; and

(d) from 0.1 to 20 weight percent of the composition of a higheralkenyl-functional organopolysiloxane gum.

The incorporation of the higher alkenyl-functional siloxane gum providesa coating composition which cures rapidly to a coating that exhibits alow COF and stable release over time. In the context of this invention,a higher alkenyl-functional polymer or gum is one which the alkenylgroups contain terminal unsaturation and have three or more carbonatoms, and the CH₂ ═CH-- groups are not attached directly to the siliconatoms of the polymer or gum.

The coating composition described herein is preferably solvent-free. Itis useful as a rapidly curable silicone release formulation. The curedformulation provides a silicone coating that gives stable release from abroad range of pressure-sensitive adhesives and exhibits a lowcoefficient of friction. This formulation is potentially useful in anyapplication in which one or both of these properties is desired and isparticularly useful where stable premium release is desired. As usedherein, premium release means a release force less than or equal toabout 0.6 newtons per decimeter (N/dm) width.

The present invention also provides a substrate bearing a cured coatingof the composition and an adhesive article that employs the coatedsubstrate. Examples of materials of these types include release liners,single and double-sided adhesive tapes, adhesive transfer tapes,labelling materials, die cuts and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The various utilities of the composition of the invention are furtherillustrated in the accompanying drawings.

FIG. 1 is a plan view of a flexible substrate which has a coating of thecomposition of the invention on one of its surfaces.

FIG. 2 is a plan view of a second embodiment of the invention in whichthe coating of the composition of the invention is present on oppositesurfaces of the substrate.

FIG. 3 is a plan view of a third embodiment of the invention in which apressure-sensitive adhesive is applied over the surface of the coatingof the composition of the embodiment of FIG. 1.

FIG. 4 is a plan view of a fourth embodiment of the invention in whichthe adhesive surface of a pressure-sensitive adhesive tape is applied tothe surface of the coating composition of the embodiment of FIG. 1.

FIG. 5 is a plan view of a fifth embodiment of the invention similar tothe embodiment illustrated in FIG. 4 except employing a two-sidedpressure-sensitive adhesive tape.

FIG. 6 is a plan view of a sixth embodiment of the invention in which apressure-sensitive adhesive is applied to the second surface of theembodiment of FIG. 1.

DETAILED DESCRIPTION

The base polymer useful in the invention typically comprises atriorganosiloxy endblocked polydiorganosiloxane polymer having aviscosity at 25° C. of from about 25 to about 5000 centipoise,preferably from about 25 to about 1000 centipoise, more preferably fromabout 25 to about 500 centipoise. The base polymer preferably comprisesR₂ SiO_(2/2) and R₃ SiO_(1/2) units, wherein each R group independentlyrepresents a saturated or ethylenically unsaturated, substituted orunsubstituted hydrocarbon radical, provided that at least two R groupscontain terminal ethylenic unsaturation. The ethylenically unsaturatedradicals are independently selected from the group consisting of thevinyl radical and higher alkenyl radicals represented by the formula--R'(CH₂)_(m) CH═CH₂ wherein R' denotes --(CH₂)_(n) -- or --(CH₂)_(p)CH═CH-- and m has the value of 1, 2, or 3; n has the value of 0, 3, or6; and p has the value of 3,4, or 5. Preferably R' denotes --(CH₂)_(n)--. Preferably the alkenyl radicals are selected from the groupconsisting of the vinyl radical and the 5-hexenyl radical. The mostpreferred saturated hydrocarbon radical methyl radical.

While the ethylenically-unsaturated organopolysiloxane is describedherein as linear and bearing only hydrocarbon radicals on silicon, it iswithin the scope and spirit of this invention to permit the presencetherein of trace amounts of non-linear siloxane units, i.e., SiO_(4/2)and RSiO_(3/2), wherein R is as described above, and trace amounts ofother silicon-bonded radicals, such as hydroxyl and alkoxyl.

The crosslinking agent useful in the invention preferably comprises anorganopolysiloxane polymer preferably having a viscosity at 25° C. ofless than 200 centipoise. Typically the crosslinking agent contains anaverage of at least three silicon-bonded hydrogen atoms per molecule.The crosslinking agent preferably comprises R₃ SiO_(1/2), HR₂ SiO_(1/2),R₂ SiO_(2/2), HRSiO_(2/2), RSiO_(3/2), and SiO_(4/4) units, wherein R isas described above. R is preferably the methyl radical.

The ratio of the number of moles of silicon-bonded hydrogen atoms in theorganohydrogenpolysiloxane crosslinking agent to the total number ofmoles of ethylenic unsaturation in the base polymer and theethylenically-unsaturated organopolysiloxane gum (described hereinafter)ranges from about 0.5:1 to about 5:1, preferably from about 0.8:1 toabout 2:1, more preferably from about 0.9:1 to about 1.5:1.

Ethylenically-unsaturated organopolysiloxanes andorganohydrogenpolysiloxanes useful in the practice of the presentinvention are well known in the art and are disclosed in such patents asAshby, U.S. Pat. No. 3,159,662; Lamoreaux, U.S. Pat. No. 3,220,972; andJoy, U.S. Pat. No. 3,410,886. Preferred ethylenically-unsaturatedorganopolysiloxanes are those containing higher alkenyl groups such asthose described by Keryk et al., U.S. Pat. No. 4,609,574. Thedisclosures of these references with respect to theethylenically-unsaturated organopolysiloxane base polymers andorganohydrogenpolysiloxane crosslinking agents disclosed therein areincorporated herein by reference.

The hydrosilylation catalyst useful in the invention is any preciousmetal-containing hydrosilylation catalyst that is effective to catalyzethe addition reaction of a compound containing at least onesilicon-bonded hydrogen atom with a compound containing ethylenicunsaturation. The catalyst preferably contains platinum or rhodium. Thecatalyst is present in an effective amount ranging from about 1 to about1000 parts of precious metal per one million pans of the composition,preferably from about 10 to about 200 parts.

Examples of useful hydrosilylation catalysts include chloroplatinicacid, platinum deposited on a substrate, platinum complexed with organicliquids, such as alcohols, aldehydes, and vinylsiloxanes, and complexesof rhodium halides. Preferably the hydrosilylation catalyst is solublein the curable silicone release composition.

Hydrosilylation catalysts are well known in the organosilicon art andneed no further delineation herein. For further details, if needed, thereader is directed to the teachings of Speier et al., U.S. Pat. No.2,823,218; Willing, U.S. Pat. No. 3,419,593; Karstedt, U.S. Pat. Nos.3,715,334 and 3,814,730; Ashby, U.S. Pat. No. 4,421,903; Lamoreaux, U.S.Pat. No. 3,220,972; Chandra et al., U.S. Pat. No. 4,603,215; Lewis, U.S.Pat. No. 4,705,765; Drahnak, U.S. Pat. Nos. 4,510,094 and 4,530,879;Eckberg, U.S. Pat. No. 4,670,531; Boardman et al., U.S. Pat. No.4,916,169; and Oxman et al., U.S. Pat. No. 5,145,886. The disclosures ofthese references with respect to the hydrosilylation catalysts disclosedtherein are incorporated herein by reference.

The organopolysiloxane gum useful in the invention is a triorganosiloxyendblocked polydiorganosiloxane polymer having an average molecularweight (as measured by gel permeation chromotography (GPC)) of at leastabout 75,000, preferably greater than about 150,000, more preferablygreater than about 300,000. It comprises R₂ SiO_(2/2) and R₃ SiO_(1/2)units, wherein each R group independently represents a saturated orethylenically unsaturated, substituted or unsubstituted hydrocarbonradical, provided that at least two R groups contain terminal ethylenicunsaturation. The ethylenically unsaturated radicals are independentlyselected from higher alkenyl radicals represented by the formula--R'(CH₂)_(m) CH═CH₂ wherein R' denotes --(CH₂)_(n) -- or --(CH₂)_(p)CH═CH-- and m has the value of 1, 2, or 3; n has the value of 0, 3, or6; and p has the value of 3, 4, or 5. Preferably R' denotes --(C₂)_(n)--. The most preferred alkenyl radical is the 5-hexenyl radical. Themost preferred saturated hydrocarbon radical is the methyl radical.

While the ethylenically-unsaturated organopolysiloxane gum is describedherein as linear and bearing only hydrocarbon radicals on silicon, it iswithin the scope and spirit of this invention to permit the presencetherein of trace amounts of non-linear siloxane units, i.e., SiO_(4/2)and RSiO_(3/2), wherein R is as described above, and trace amounts ofother silicon-bonded radicals, such as hydroxyl and alkoxyl.

The number of ethylenically unsaturated radicals in theorganopolysiloxane gum preferably ranges from a minimum of two permolecule to a maximum of about 5.0 mole percent of the average totalnumber of silicon atoms present in the molecule. Preferably the numberof ethylenically unsaturated radicals in the gum comprises less thanabout 2.0 mole percent, more preferably less than about 0.5 molepercent, and-still more preferably less than about 0.2 mole percent ofthe average total number of silicon atoms.

The organopolysiloxane gum is present in an effective amount, preferablyfrom about 0.5 to about 10.0 weight percent, more preferably from about1.0 to about 5.0 weight percent of the composition. More than 20 weightpercent of the gum may be used if desired. However, the viscosity of thecomposition is generally too high to coat conveniently when such a levelof the gum is used.

Mixtures of one or more of each of the base polymers, the crosslinkingagent, the hydrosilylation catalyst and the higher alkenyl-functionalgum may be employed in the practice of the invention if desired.Additionally, the coating composition of the invention may contain otheradjuvants if desired. These materials should not adversely interferewith the curing of the composition.

For example, the release characteristics of the composition may bemodified by the use of controlled release additives. When applied to onesurface of a substrate and cured, such coatings provide a surface havinghigher release levels than unmodified coatings. The opposite surface ofthe substrate may then be coated with an unmodified composition toprovide a differential release liner. Such liners preferably have arelease differential of at least 10% between the two surfaces. They areespecially useful with adhesive transfer tapes.

Other additives may also be included. Specifically, the inclusion ofpigments, theology control additives, substrate-adhesion promoters, andadjuvants to control substrate-penetration by the coating composition isalso contemplated within the scope of this invention.

The compositions of the present invention may also optionally contain aneffective amount of an inhibitor for the metal hydrosilylation catalyst.As is well known, preferred inhibitors reduce the activity of the metalcatalyst at room temperature thus increasing the time period duringwhich the composition may be used at room temperature while stillallowing rapid cure at elevated temperatures.

Hydrosilylation catalyst inhibitors are well known in the art andinclude such compounds as pyridine (U.S. Pat. No. 3,188,299),acrylonitrile (U.S. Pat. No. 3,344,111), 2-methyl-3-buten-2-ol (U.S.Pat. No. 3,445,420), organic phosphines and phosphites (U.S. Pat. Nos.3,989,666 and 4,336,364), benzotriazole (U.S. Pat. No. 3,192,181),organic sulfoxides (U.S. Pat. No. 3,453,234), aminofunctional siloxanes(U.S. Pat. No. 3,723,567), ethylenically unsaturated isocyanurates (U.S.Pat. No. 3,882,038), olefinic siloxanes (U.S. Pat. Nos. 3,933,880,3,989,666, and 3,989,667), alkenynes (U.S. Pat. No. 4,559,396),unsaturated carboxylic esters (U.S. Pat. Nos. 4,256,870, 4,347,346,4,774,111, and 5,036, 117), and unsaturated carboxylic amides (U.S. Pat.No. 4,337,332).

The compositions of the invention may be prepared by mixing the desiredamounts of the above described components and any additional componentsin any suitable manner such as by stirring, blending and/or tumbling andin any suitable order. Preferably the organohydrogen polysiloxanecrosslinking agent is added last.

The compositions may also be prepared by combining two non-curingcompositions which, when mixed in proper proportions, will give rise tothe curable coating composition. For example, one of the non-curingcompositions may comprise a portion of the olefinic polydiorganosiloxanebase polymer, a portion of the organopolysiloxane gum and theorganohydrogenpolysiloxane crosslinking agent and another of thenon-curing compositions comprises the balance of the olefinicpolydiorganosiloxane base polymer, the balance of the organopolysiloxanegum, an inhibitor and the hydrosilylation catalyst. Alternatively, oneof the non-curing compositions may comprise all of the components exceptthe organohydrogenpolysiloxane crosslinking agent, which constitutesanother non-curing composition to be mixed with the first non-curingcomposition at the proper time.

The compositions of the invention may be applied as a continuous layerto a substrate. Alternatively, it may be applied as a discontinuouslayer such as in the form of one or more stripes, as individual islands,in a random or ordered pattern, etc.

Any solid substrate may be treated by the compositions of this inventionto provide release of adhesive materials therefrom. Examples of suitablesubstrates include rubber, cellulosic materials, such as paper,cardboard, and wood; metals, such as aluminum, iron, and steel;siliceous materials, such as ceramics, glass, and concrete; andsynthetic polymers, such as polyester, polyepoxide, and polyethylene. Toassure proper curing and adhesion of the silicone coating, the substrateshould be clean and free of materials which undesirably inhibit the cureof the release composition, such as materials containing certain amines,mercaptans and phosphines.

The curable coating compositions of this invention are particularlyuseful in preparing a laminate of substrate and an adhesive wherein theadhesive will release from the substrate. This laminate may be preparedby (a) coating the curable silicone composition of the present inventionon the surface of the substrate either in a continuous process or at alater time; (b) curing the silicone release composition with aneffective amount of heat and/or radiation; (c) applying an adhesive tothe silicone surface of the substrate.

The curable silicone composition of this invention may be coated on thesurface of the substrate by any of the well known methods for coatingflexible substrates such as brushing, dipping, spraying, five smoothroll coating, knife over roll, reverse roll, and gravure. Any suitableamount of coating material may be applied to the substrate. Generally, athin coating of the silicone release composition is employed. Typicallythe coating weight is approximately 0.1 to 5.0 grams of the curablesilicone composition per square meter of coated surface with about onegram per square meter of coated surface being preferred.

Curing of the composition of the invention can be accomplished by avariety of methods. Typically, they are accomplished via a techniquethat is consistent with the catalyst used. Thus, for example, curing canbe accomplished by exposure to heat, ultraviolet light, visible light,electron beam radiation, gamma radiation and the like. When heat is usedto cure the composition, the curing mechanism can be initiated in anyconvenient way such as by infrared lamps, by radiation, or by forced airoven which is often most suitable. Curing can be accomplished at anyconvenient temperature, but typically it is most advantageous to usetemperatures above room temperature such as from 50° C. to 150° C. withtemperatures of from 70° C. to 120° C. being preferred to provide rapidcure while conserving heat energy.

When UV light is used to initiate curing, it is preferably done byexposure to medium pressure ultraviolet lights. When gamma radiation isused, it is generally not necessary to employ a catalyst.

In preparing an article of the invention, an adhesive may be laminatedto the cured release composition of the invention. Alternatively, it maybe applied in-line to the cured silicone surface of the substrate. Byin-line, it is meant that the adhesive is applied to the silicone coatedsurface of the substrate within a short time after the curing of thesilicone, and without reeling and storing the silicone coated substrateprior to the adhesive application step. Alternatively, the adhesive maybe applied at a later time rather than in-line by storing the coatedsubstrate.

The adhesive may be applied by any of the well known methods such as bycoating hot melt adhesives, solutions of adhesive in solvent or water,or dispersions of adhesives, or by pressing an adhesive film supportedon a sheet material to either surface of the silicone coated surface.

The type of adhesive material used in the process of this invention isnot critical and any of the well known materials can be used.Pressure-sensitive adhesives are a particularly useful class ofadhesives. Such adhesives include acrylic adhesives, block copolymeradhesives, diene polymer based adhesives and the like. A description ofuseful pressure-sensitive adhesives may be found in Encyclopedia ofPolymer Science and Engineering, Vol. 13, Wiley-Interscience Publishers(New York, 1988). Additional description of useful pressure-sensitiveadhesives may be found in Encyclopedia of Polymer Science andTechnology, Vol. 1, Interscience Publishers (New York, 1964).

The present invention may be used in a variety of applications. Examplesof these utilities are shown in FIGS. 1-6 in which like referencenumbers refer to the same elements.

FIG. 1 shows a cross-sectional view of a substrate 10 bearing a layer 12of the composition of the invention on one of its surfaces. Preferablysubstrate 10 is a thin flexible material and coating 12 is cured. Theembodiment shown in FIG. 1 is useful as a release liner and as a backingfor a pressure sensitive adhesive.

FIG. 2 shows a cross-sectional view of a variation of the embodimentshown in FIG. 1. In FIG. 2, substrate 10 has a layer 12 of thecomposition of the invention on both of its surfaces. This embodiment isuseful whenever a two-sided release capability is desired. It should benoted that one of the surfaces of the embodiment of FIG. 2 could also becoated with a layer of a pressure-sensitive adhesive to provide atransfer tape.

FIG. 3 shows a cross-sectional view of a substrate 10 which has a layer12 of the composition of the invention on one of its surfaces. A layer14 of a pressure-sensitive adhesive is provided on layer 12. Thisembodiment is useful as an adhesive tape.

FIG. 4 shows a cross-sectional view of the embodiment of FIG. 1 to whichan adhesive product comprising an adhesive layer 14 and a backing 16 hasbeen secured. This embodiment is useful as a label stock, die cutproduct, decorative applique, single-sided tape, surgical drape, diapertape, etc.

FIG. 5 shows a cross-sectional view of a double-coated adhesive tapesecured to a removable liner. The double-coated adhesive tape comprisesa backing 16 having a layer 14 of adhesive secured to both of itssurfaces. The liner comprises a substrate 10 which has a layer 12 of thecomposition of the invention on each of its surfaces. The adhesive layer14 is in contact with one of the layers 12 of the removable liner.

FIG. 6 shows an adhesive tape. The tape comprises a substrate 10 whichhas a layer 14 of pressure-sensitive adhesive on one of its surfaces anda layer 12 of the composition of the invention on the other of itssurfaces.

Each of the embodiments shown in the Figures may be wound uponthemselves in a roll form. In the case of the embodiments of FIGS. 3, 5and 6, it may be desirable to provide a release surface on the exposedsurface of the substrate 10 to prevent the adhesive from adhering tosubstrate 10. This may be accomplished in a number of ways. For example,the exposed surface of substrate 10 may be treated with a particulatematerial. Alternatively, it may have a low adhesion backsize material onit to prevent adhesion. Low adhesion backsize materials useful hereininclude polyolefins; cured silicones; polyurethanes; blends of siliconeswith polyolefins; blends of fluorochemicals with polyolefins;polyurethanes; or fluorochemicals grafted to polyolefins or similarpolymers; and the like. Low adhesion backsize materials are described inthe literature. See for example, U.S. Pat. Nos. 2,532,011; 2,607,711;and 3,318,852.

The following examples are presented to illustrate the invention tothose skilled in the art and should not be construed as limiting theinvention, which is properly delineated in the appended claims. Allproportions by parts or percents are by weight unless otherwise stated.

Base Polymers

A series of base polymers was employed in the following examples. Thesepolymers were prepared by the following representative procedure.

In a half-gallon polyethylene bottle were combined 925.3 g ofoctamethylcyclotetrasiloxane (3.119 mol), 66.4 g of1,3,5,7-tetramethylcyclotetrasiloxane (0.276 mol), 30.9 g of1,1,3,3-tetramethyldisiloxane (0.230 mol), 5.1 g of activated carbon and1.0 g of concentrated sulfuric acid. The mixture was agitated at roomtemperature for 24 hr and filtered. Volatiles were separated from thefiltrate at 200° C. using a thin film evaporator to give 884.0 g of theintermediate product. A 5-L four-necked round-bottomed flask equippedfor mechanical stirring was charged with 300.0 g of 1,5-hexadiene (3.652mol) and 200 mL of heptane and fitted with a 1-L addition funnel and areflux condenser. The addition funnel was charged with a solution of theintermediate product and 250 mL of heptane. A 20 mL syringe was chargedwith a solution 200 mg of a precious metal-containing catalyst (15 wt %Pt in ViMe₂ SiOSiMe₂ Vi, 25 ppm) in 12 mL of heptane and fitted to asyringe pump. A length of 20-gauge Teflon™ tubing was attached to thesyringe leading into the reaction mixture through a rubber septum fittedto the fourth neck of the flask. The reaction mixture was heated to 60°C. stirring was begun, and the solutions of the intermediate product andplatinum catalyst were added simultaneously, the former dropwise, thelatter at a rate of 1.7 mL/hr. After the addition of the intermediateproduct was complete, the addition of the platinum catalyst was stopped,and the reaction mixture was stirred at 60° C. for 3 hr, when analysisof an aliquot by IR indicated complete consumption of silane (absence ofSi--H band at 2160 cm⁻¹). In order to adsorb the platinum catalyst, 15 gof colloidal clay (Attagel®-50) was added, and the mixture was stirredat room temperature overnight. The reaction mixture was filtered, andvolatiles were separated on a rotary evaporator at 0.5 mm and 60° C. togive 975.1 g of product base polymer G. ¹ H and ²⁹ Si NMR analysis ofthe product indicated a polymer with the average structure(5-hexenyl)Me₂ SiO[Si(5-hexenyl)MeO]₄.6 [SiMe₂ O]₅₇.7 SiMe₂ (5-hexenyl).

Base polymers C, D, E, F, and H were prepared in an analogous fashion.

¹ H and ²⁹ Si NMR analysis of Syl-Off™ 7681 and 7686, (commerciallyavailable from Dow Corning Corporation) indicated that theseformulations contained base polymers A and B, respectively.

The structures of base polymers A through H are given below:

    ______________________________________                                                                          mol                                         Poly-                             %                                           mer   Structure                   Hex                                         ______________________________________                                        A     HexMe.sub.2 SiO(SiMe.sub.2 O).sub.119.0 SiMeHex                                                           1.7                                         B     HexMe.sub.2 SiO(SiMe.sub.2 O).sub.176.8 (SiMeHexO).sub.2.3 SiMe.sub.          2 Hex                       2.4                                         C     HexMe.sub.2 SiO(SiMe.sub.2 O).sub.76.4 (SiMeHexO).sub.1.6 SiMe.sub.2           Hex                        4.5                                         D     HexMe.sub.2 SiO(SiMe.sub.2 O).sub.65.1 (SiMeHexO).sub.1.7 SiMe.sub.2           Hex                        5.4                                         E     HexMe.sub.2 SiO(SiMe.sub.2 O).sub.69.8 (SiMeHeXO).sub.3.3 SiMe.sub.2           Hex                        7.1                                         F     HexMe.sub.2 SiO(SiMe.sub.2 O).sub.79.1 (SiMeHexO).sub.5.9 SiMe.sub.2           Hex                        9.1                                         G     HexMe.sub.2 SiO(SiMe.sub.2 O).sub.57.7 (SiMeHeXO).sub.4.6 SiMe.sub.2           Hex                        10.2                                        H     HexMe.sub.2 SiO(SiMe.sub.2 O).sub.54.0 (SiMeHexO).sub.5.6 SiMe.sub.2           Hex                        12.3                                        ______________________________________                                    

In these structures,

Hex=5-hexenyl (i.e., CH₂ ═CH--CH₂ --CH₂ --CH₂ --CH₂ --)

Me=methyl (i.e., CH₃ --)

Crosslinking Agent

The crosslinking agent employed in the examples comprised a 1:1 byweight mixture of

    Me.sub.3 SiO(Me.sub.2 SiO).sub.15 (MeHSiO).sub.25 SiMe.sub.3

and

    Me.sub.3 SiO(MeHSiO).sub.40 SiMe.sub.3.

These materials are respectively Syl-Off™ 7678 and 7048 (commerciallyavailable from Dow Coming Corporation).

Catalyst

The catalyst employed in the examples is Syl-Off™ 7127 (commerciallyavailable from Dow Corning Corporation).

Inhibitor

The inhibitor employed in the examples was a 70:30 mixture by weight ofdiethyl fumarate and benzyl alcohol, each commercially available fromAldrich Chemical Company.

Organopolysiloxane Gum

A series of ethylenically unsaturated gums was employed in the followingexamples. They were prepared by the following representative procedure.

In a 500 mL three necked resin flask equipped for mechanical stirringwere combined 230.0 g of octamethylcyclotetrasiloxane (775 mmol), 1.48 gof a (5-hexenyl)dimethylsiloxy endblocked polydimethylsiloxane havingthe average structure (5-hexenyl)Me₂ SiO(SiMe₂ O)₁₄₅ SiMe₂(5-hexenyl)(0.13 mmol, prepared in a manner analogous to that describedfor the preparation of polymer G above), and 14 mg of potassiumhydroxide (0.25 mmol). The reaction mixture was heated with stirring to160° C. under an argon atmosphere. Initially rapid, the rate of stirringwas decreased as the viscosity of the reaction mixture increased. After48 hr the base catalyst was quenched by passing a stream of carbondioxide through the hot reaction mixture with continued stirring forapproximately 5 hr. The product was transferred to an open aluminum pan,spread into a 5 mm thick layer, and devolatilized in a forced air ovenat 150° C. for 6 hr. Analysis of the final product G7 by GPC indicated apeak molecular weight of 473,000, corresponding to an average degree ofpolymerization (dp) of 6390.

The resulting gums had the formula

    HexMe.sub.2 SiO(SiMe.sub.2 O).sub.x (SiMeHexO).sub.y SiMe.sub.2 Hex

The following organopolysiloxane gums were prepared:

    ______________________________________                                        Gum          dp (x + y)                                                                              Mol % Hex                                              ______________________________________                                        G1           10200     0.16                                                   G2           1210      0.17                                                   G3           1800      0.11                                                   G4           2820      0.07                                                   G5           3490      0.06                                                   G6           8560      0.05                                                   G7           6390      0.03                                                   ______________________________________                                    

The dp was determined by dividing the peak molecular weight from GPCanalysis by 74. Higher values of dp indicate higher molecular weightmaterials.

Coating Methods

Compositions were prepared by combining base polymer, organopolysiloxanegum, catalyst, inhibitor, and crosslinking agent as described in thefollowing examples except for base polymers A and B which were receivedpremixed with catalyst and inhibitor. The resultant compositions werecoated onto polyethylene-coated Kraft paper using a five smooth rollcoater set up in-line with a 10 ft (3.05 m) impinged air oven. Thecoating compositions were applied at a coating weight of about 1 g/m²and cured at 116° C. (240° F.) at 12 seconds oven residence time (50ft/min (15.24 m/min)) through the oven.

The following test methods were employed in the examples:

Coefficient of Friction

The coefficients of friction of the cured silicone coatings weredetermined by the following procedure, based on ASTM D 1894-63,sub-procedure A. An approximately 10×6 inch (25×15 cm) area of thesilicone-coated substrate was adhered to the platform of an IMASSSlip/Peel tester (model SP-102B-3M90) such that the silicone-coatedsurface was exposed. Care was taken to insure that the silicone surfaceof the silicone-coated substrate was untouched, uncontaminated, flat,and free of wrinkles. The sample surface and the friction sled (modelSP-101038, wrapped with 3.2 mm thick, medium-density foam rubber) wereblown with compressed air to remove any loose dust or debris, thefriction sled was placed on the silicone surface, and the chain attachedto the sled was affixed to the force transducer of the IMASS Slip/Peeltester. The platform of the IMASS Slip/Peel tester was set in motion ata speed of 6 in/min (15 cm/min), thereby dragging the friction sledacross the silicone surface. The instrument calculated and reported theaverage kinetic frictional force, omitting the static frictional force.The kinetic coefficient of friction was obtained by dividing the kineticfrictional force by the weight of the friction sled.

Release Values

The release values of the cured silicone coatings were determined by thefollowing procedure. A heptane-methyl ethyl ketone solution ofpressure-sensitive adhesive comprising isooctyl acrylate (95.5 wt%)-acrylic acid (4.5 wt %) copolymer, as described in Example 5 of U.S.Pat. No. Re. 24,906 was applied to the cured silicone coating and driedfor 5 minutes in a circulating air oven to give a dry coating weight of32 g/m². A biaxially oriented film of polyethylene terephthalate (PET,38 micrometers thick) was pressed against the surface of the coating toproduce a laminate consisting of a pressure-sensitive adhesive tape anda silicone-coated substrate. The laminate was cut into 1.0×10 inch(2.5×25 cm) strips. The release force is the force required to pull thePET film with the adhesive attached thereto (i.e., a pressure-sensitiveadhesive tape) away from the silicone coated substrate at an angle of180° and a pulling speed of 90 in/min (230 cm/min).

Readhesion Value

The readhesion values of the pressure-sensitive adhesive tapes weredetermined by the following procedure. The pressure-sensitive adhesivetapes, as removed from the silicone coated surface, were applied to thesurface of a clean stainless steel plate.. The readhesion value is theforce required to pull the tape from the steel surface at an angle of180° and a pulling speed of 12 in/min(30 cm/min).

Release and readhesion data were obtained using an IMASS Slip/Peeltester (model SP-102B-3M90). Release and readhesion values weredetermined both on samples that had stood undisturbed at roomtemperature for three days (initial) and on samples that had been heatedat 70° C. for three days (aged).

EXAMPLES 1-7

To each of seven 60.0 g samples of Sly-Off™ 7686(containing base polymerB) was added 1.80 g (3.0 wt %) of one of the organopolysiloxane gums G1through G7. The resultant mixtures were agitated on a mechanical shakeruntil they were homogeneous (approximately 48 hr). To a 30.0 g portionof each of these mixtures was added 0.90 g of crosslinking agent. Theseven resultant compositions were coated, cured, and tested as describedabove, and the results are recorded in Table I.

                  TABLE I                                                         ______________________________________                                                                    Readhesion                                                    Release (N/dm)  (N/dm)                                            Ex.   Gum    COF      Initial                                                                             Aged    Initial                                                                             Aged                                ______________________________________                                        1     G1     0.29     0.31  0.39    36.1  33.9                                2     G2     0.26     0.23  0.35    37.2  33.9                                3     G3     0.23     0.27  0.35    40.5  33.9                                4     G4     0.23     0.27  0.35    42.7  41.6                                5     G5     0.18     0.27  0.35    44.8  30.6                                6     G6     0.23     0.31  0.39    35    35                                  7     G7     0.17     0.23  0.35    39.4  33.9                                ______________________________________                                    

EXAMPLES 8-13 and C1

To a 60.0 g sample of base polymer D was added 0.31 g of theorganopolysiloxane gum G7, the resultant mixture was agitated on amechanical shaker until it was homogeneous (approximately 48 h), and0.78 g of inhibitor and 1.40 g of catalyst were added to give a mixturecontaining 0.5 wt % of the organopolysiloxane gum G7. To a 30.0 gportion of this mixture was added 1.78 g of crosslinking agent.Similarly prepared were mixtures of 60.0 g of base polymer D, 0.62 g ofgum, 0.78 g of inhibitor, and 1.40 g of catalyst (1.0 wt % gum), to 30.0g of which mixture was added 1.77 g of crosslinking agent; 70.0 g ofbase polymer D, 2.18 g of gum, 0.91 g of inhibitor, and 1.64 g ofcatalyst (3.0 wt % gum), to 30 g of which mixture was added 1.74 g ofcrosslinking agent; 60.0 g of base polymer D, 3.13 g of gum, 0.78 g ofinhibitor, and 1.40 g of catalyst (5.0 wt % gum), to 30.0 g of whichmixture was added 1.71 g of crosslinking agent; 60.0 g of base polymerD, 6.23 g of gum, 0.78 g of inhibitor, and 1.40 g of catalyst (10.0 wt %gum), to 30.0 g of which mixture was added 1.63 g of crosslinking agent;60.0 g of base polymer D, 12.47 g of gum, 0.78 g of inhibitor, and 1.40g of catalyst (20.0 wt % gum), to 30.0 g of which mixture was added 1.49g of crosslinking agent. A control formulation (Comparative Example 1)was prepared by combining 30.0 g of base polymer D, 0.39 g of inhibitor,0.70 g of catalyst, and 1.79 g of crosslinking agent. The eightresultant compositions were coated, cured, and tested as describedabove, and the results are recorded in Table II.

                  TABLE II                                                        ______________________________________                                                                 Release   Readhesion                                 Wt %     Viscosity       (N/dm)    (N/dm)                                     Ex.  G7      (Cps)    COF  Initial                                                                             Aged  Initial                                                                             Aged                             ______________________________________                                        C1   0.0     180      0.45 0.27  0.62  29.5  31.7                              8   0.5     200      0.14 0.31  0.58  29.5  30.6                              9   1.0     250      0.11 0.31  0.54  30.6  30.6                             10   3.0     460      0.09 0.31  0.5   30.6  30.6                             11   5.0     870      0.09 0.31  0.5   29.5  30.6                             12   10.0    3760     0.12 0.35  0.5   29.5  29.5                             13   20.0    26600    0.16 0.31  0.5   31.7  29.5                             ______________________________________                                    

EXAMPLES 14-17 and C2

To each of four separate 60.0 g samples of Syl-Off™ 7686(containing basepolymer B) was added either 1.6 g (1 wt %), 1.8 g (3 wt %), 3.0 g (5 wt%) or 6.0 g (10 wt %) of the organopolysiloxane gum G7, and theresultant mixtures were agitated on a mechanical shaker until they werehomogeneous (approximately 48 h). To a 30.0 g portion of each of thesemixtures was added 0.92 g, 0.90 g, 0.88 g, and 0.84 g, respectively, ofcrosslinking agent. A control formulation (Comparative Example 2) wasprepared by combining 30.0 g of Syl-Off™ 7686 and 0.93 g of crosslinkingagent. The five resultant compositions were coated, cured, and tested asdescribed above, and the results are recorded in Table III.

                  TABLE III                                                       ______________________________________                                                                 Release   Readhesion                                 Wt %     Viscosity       (N/dm)    (N/dm)                                     Ex.  G7      (Cps)    COF  Initial                                                                             Aged  Initial                                                                             Aged                             ______________________________________                                        C2   0.0      450     0.43 0.23  0.31  35    38.3                             14   1.0      600     0.21 0.27  0.42  37.2  36.1                             15   3.0     1090     0.17 0.23  0.35  39.4  33.9                             16   5.0     1900     0.17 0.31  0.42  38.3  32.8                             17   10.0    6880     0.15 0.27  0.42  37.2  35.0                             ______________________________________                                    

EXAMPLES 18-25

To a 75.0 g sample of base polymer D were added 0.98 g of inhibitor,1.95 g of catalyst, and 2.34 g of the organopolysiloxane gum G7, and theresultant mixture containing 3 wt % of the organopolysiloxane gum G7 wasagitated on a mechanical shaker until it was homogeneous (approximately48 h). To a 50.0 g portion of this mixture was added 3.10 g ofcrosslinking agent. Similarly prepared were mixtures of 60.0 g of basepolymer C, 1.87 g of gum, 0.78 g of inhibitor, and 1.40 g of catalyst (3wt % gum), to which mixture was added 1.77 g of crosslinking agent; 75.0g of base polymer E, 2.34 g of gum, 0.98 g of inhibitor, and 1.95 g ofcatalyst (3 wt % gum), to 50.0 g of which mixture was added 3.98 g ofcrosslinking agent; 100.0 g of base polymer H, 3.00 g of gum, 1.30 g ofinhibitor, and 2.60 g of catalyst (3 wt % gum), to 50.0 g of whichmixture was added 6.66 g of crosslinking agent. To 200.0 g of basepolymer F were added 2.60 g of inhibitor and 5.2 g of catalyst. To a75.0 g portion of this mixture was added 2.25 g of theorganopolysiloxane gum G7 (3 wt % gum), and the resultant mixture wasagitated on a mechanical shaker until it was homogeneous (approximately48 h). To a 50.0 g portion of this mixture was added 5.40 g ofcrosslinking agent. Similarly prepared was a mixture of 200.0 g of basepolymer G, 2.60 g of inhibitor, and 5.2 g of catalyst, to 75.0 g ofwhich mixture was added 2.25 g of gum G7 (3 wt % gum). To a 50.0 gportion of this mixture was added 5.40 g of crosslinking agent. To a75.0 g sample of Syl-Off™ 7681(containing base polymer A) was added 2.25g of organopolysiloxane gum G7 (3 wt % gum), and the resulting mixturewas agitated on a mechanical shaker until it was homogeneous(approximately 48 h). To a 30.0 g portion of this mixture was added 0.67g of crosslinking agent. Similarly prepared was a mixture of 75.0 g ofSyl-Off™ 7686 (containing base polymer B) and 2.25 g of gum G7 (3 wt %gum), to 30.0 g of which mixture was added 0.92 g of crosslinking agent.The eight resultant compositions were coated, cured, and tested asdescribed above, and the results are recorded in Table IV.

                  TABLE IV                                                        ______________________________________                                                                          Readhesion                                  Base               Release (N/dm) (N/dm)                                      Ex.   Polymer  COF     Initial                                                                             Aged   Initial                                                                             Aged                                ______________________________________                                        18    A        0.23    0.27  0.42   35    35                                  19    B        0.17    0.23  0.35   39.4  33.9                                20    C        0.2     0.35  0.39   33.9  30.6                                21    D        0.08    0.35  0.5    29.5  28.4                                22    E        0.07    0.39  0.5    30.6  27.3                                23    F        0.06    0.46  0.62   30.6  28.4                                24    G        0.07    0.42  0.5    32.8  25.2                                25    H        0.08    0.46  0.62   35    29.5                                ______________________________________                                    

We claim:
 1. A curable coating composition comprising:(a) anethylenically unsaturated organopolysiloxane base polymer having aviscosity of from 25 to about 5,000 centipoise at 25° C. (b) anorganohydrogenpolysiloxane crosslinking agent; (c) an effective amountof a precious metal containing hydrosilylation catalyst; and (d) from0.1 to 20 weight percent of the composition of a higheralkenyl-functional organopolysiloxane gum;wherein said gum is atriorganosiloxy endblocked polydiorganosiloxane polymer having anaverage molecular weight of at least 75,000 as measured by gelpermeation chromatography and having the formula comprising R₂ SiO_(2/2)and R₃ SiO_(1/2) units wherein each R group is independently selectedfrom the group consisting of saturated and ethylenically unsaturated,hydrocarbon radicals, provided that at least two of said R groups arehigher alkenyl radicals.
 2. The curable coating composition of claim 1wherein the higher alkenyl radicals are represented by the formula--R'(CH₂)_(m) CH═CH₂ wherein R' is selected from the group consisting of--(CH₂)_(n) -- and --(CH₂)_(p) CH═CH-- whereinm is 1, 2, or 3; n is 0,3, or 6; and p is 3,4, or
 5. 3. The curable coating composition of claim2 wherein the higher alkenyl radicals are 5-hexenyl radicals.
 4. Thecurable coating composition of claim 3 wherein the saturated hydrocarbonradical are methyl radicals.
 5. The curable coating composition of claim1 wherein the higher alkenyl radicals in the gum are present at a levelup to 5 mole percent of the average total number of moles of siliconatoms per one mole of gum.
 6. The curable coating composition of claim 5wherein the higher alkenyl radicals in the gum are present at a levelless than 2 mole percent of the average total number of moles of siliconatoms per one mole of gum.
 7. The curable coating composition of claim 6wherein the higher alkenyl radicals in the gum are present at a levelless than 0.5 mole percent of the average total number of moles ofsilicon atoms per one mole of gum.
 8. The curable coating composition ofclaim 7 wherein the higher alkenyl radicals in the gum are present at alevel than 0.2 mole percent of the average total number of moles ofsilicon atoms per one mole of gum.
 9. The cured product of thecomposition of claim
 1. 10. The cured product of claim 1 having acoefficient of friction that is lower than that of a cured productobtained from a mixture comprising components (a), (b) and (c), butomitting component (d).
 11. The curable coating composition of claim 1containing from 0.5 to 10 weight percent of the composition of thehigher alkenyl-functional organopolysiloxane gum.
 12. The curablecoating composition of claim 1 containing from 1 to 5 weight percent ofthe composition of the higher alkenyl-functional organopolysiloxane gum.